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Burning Wood Efficiently

Burning Wood Efficiently

Posted by jasonon Oct 08, 2018
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The knowledge and skills needed to operate a wood burning system properly must be learned and practiced getting them right. By practicing and mastering the techniques offered here, you will reduce the amount of wood you burn to heat your home, reduce smoke pollution outside and inside the house, and increase the convenience and pleasure of burning wood.

The Basics: As firewood burns, it goes through three phases.

Evaporation of water: Up to half the weight of a freshly cut log is water. After proper seasoning the water content is reduced to less than 20 per cent. As the wood is heated in the firebox, this water boils off, consuming heat energy in the process. The wetter the wood, the more heat energy is consumed. That is why wet firewood hisses and sizzles and is hard to burn while properly seasoned wood ignites and burns easily.

The emission of smoke: As the wood heats up above the boiling point of water, it starts to smoke. The smoke is the visible result of the breakdown of the solid wood as it vaporizes into a cloud of combustible gases and tar droplets. The smoke will burn if the temperature is high enough and oxygen is present. When the smoke burns, it produces the bright flames that are characteristic of wood combustion. Smoke that does not burn in the firebox is released into the chimney where it will either condense as creosote deposits or vent to the outdoors as air pollution. Unburned smoke represents an efficiency loss because it contains a large part of the total energy in the wood.

The charcoal phase: As the fire progresses and most of the gases and tars have vaporized out of the wood, charcoal remains. Charcoal is almost entirely carbon and burns with a red glow and very little flame or smoke. Charcoal is a good fuel that burns easily. However, a charcoal fire releases carbon monoxide which is a poisonous gas, so even though it is not smoky, the exhaust must be completely vented to outdoors. In practice, all three phases of wood combustion can happen at the same time. The wood gases can be flaming, and the edges of the pieces can be glowing red as charcoal burns, while water in the core of the piece is still evaporating. The challenge in burning wood effectively is to boil off the water content quickly and make sure the smoke burns with bright flames before it leaves the firebox.

Please Note: The suggestions offered here are general and apply to many appliances used in Canada. However, some combustion systems, notably catalytic systems and masonry heaters, require special firing techniques. Instructions for these techniques will be found in the manufacturer's operation manual. If the manual for the appliance has detailed firing instructions, they should be followed.

Starting a New Fire: The first step in building a fire is to find out where the combustion air enters the firebox. For most modern stoves and fireplaces with glass doors, much of the air enters the firebox through a narrow strip above and behind the glass panel. This “air wash” flows down like a curtain to the base of the glass and keeps the tarry smoke from sticking to it. The air reaches the fire at coal bed level. Most older stoves without a glass air-wash system will have an air inlet towards the bottom of the loading door. Whether you have a new stove with glass air-wash or an older stove without it, the front of the firebox, just inside the loading door is normally where you would light the fire so that it gets plenty of air. This location is also the right ignition point for most wood-fired furnaces and boilers. There are several ways to start a wood fire successfully, and every user develops his or her own method after some trial and error. Regardless of the method you choose, the goal should be a fire that ignites quickly and builds to full intensity without smoldering. For a fire to burn with minimal smoke, its surroundings must be hot, so the first step in a clean burn is to get a hot fire burning to heat up the firebrick and metal parts of the firebox. Here are four ways to build a wood fire: using fire starters, the bottom up fire, the top-down fire and the in-between fire.

Using fire starters: Fire starters are usually made from a mixture of sawdust and wax, these can be either purchased or homemade. A few small pieces of fire starter are placed among pieces of kindling and then ignited. This is an easy way to light a fire because, as long as enough split kindling pieces are placed close enough to the burning fire starters, the fire will reliably catch. Set the air control to fully open. Depending on the stove design and chimney arrangement, you may need to leave the door open a crack until the fire catches. Since leaving the door open slightly for more than a few minutes can lead to dangerously high temperatures, never leave the stove unattended while starting the fire.

The bottom-up fire:This is the traditional way to build a fire. Crumple as many as 10 sheets of newspaper and put them in the firebox. The amount of newspaper you need will depend on the firebox size, and the dryness and fineness of the kindling you are using. The drier and finer the kindling, the less newspaper you need. Many people make the mistake of using too little newspaper. Be generous with the newspaper and you will have more success. Hold the paper down with 10 to 15 pieces of finely split, dry kindling. Softwoods, such as cedar and pine, make good kindling. Ideally, the kindling should be placed on and behind the newspaper so that the combustion air reaches the newspaper first where you light it. It is also a good idea to add two or three very small pieces of firewood to the kindling load before lighting. Set the air control to fully open, light

the newspaper and close the door or leave it open a crack. The disadvantages of the traditional method are that once the paper burns away, the fire can collapse and begin to smolder, and even if the fire doesn’t collapse, it is necessary to open the loading door again once or twice to add larger pieces until a full fire is built.

The top-down fire: The top-down method of building a wood fire is becoming more popular. To build a top-down fire, reverse the traditional method. Place three or four small pieces of split firewood on the floor of the firebox, and put a few pieces of kindling on the logs and some finely split kindling on those. Crumpled sheets of newspaper can be used, but they tend to roll around and fall off the fuel as they burn. A better method is to roll up a full sheet of newspaper from corner to corner and tie a knot in it. Use four or five newspaper knots on and around the kindling. Set the air control to fully open, light the newspaper and close the loading door. The advantages of the top-down fire building method are minimal start-up smoke, no chance that the fire will collapse and smother itself, and no need to open the loading door to add larger pieces once the kindling fire is established.

The in-between fire: The in-between fire is another common approach. Lay two medium-sized pieces of split firewood on the floor of the firebox. Place some crumpled newspaper between the pieces and lay up to a dozen pieces of finely split kindling across the two logs. When you light the newspaper the kindling catches easily and can’t collapse before the two logs catch. If you have problems getting your fires to start, the cause is likely that either the wood is not split finely enough or it is too wet.

Rekindling a Fire from Charcoal: For most wood-burning appliances, the live coals that remain after the fire has burned down are found at the back of the firebox furthest from the air supply. This is the time to clear excess ash from the firebox. Before disturbing the remaining charcoal, remove a small amount of ash from the front of the firebox. Now rake the live coals forward to just inside the loading door. If only a small amount of charcoal remains, you will have to start with kindling. If you have a good quantity of glowing charcoal to work with, place at least three, and preferably more than five pieces of firewood on and behind the charcoal. Open the air inlets fully and close the door. If everything is just right, you should expect instant ignition of the new load. The bottom pieces may start to flame before you get the door closed. Allow the fire to burn until the firebox is full of bright turbulent flames and the wood is charred black. This usually takes between five and 20 minutes, depending on the size of the pieces and the moisture content of the wood. When the wood is charred, you can reduce the air setting to produce the amount of heat and length of burn you desire. If the wood stops flaming, open the air control again and let the fire burn longer until the firebox heats up fully. You may want to try

reducing the air control setting in two or three stages. The result will be less smoke because the fire will not have to recover from the single, large reduction in air supply. The most important rule is: NEVER LET THE FIRE SMOULDER. As long as there is solid wood in the firebox, there should be active flames. Without flames the smoke will escape unburned, both reducing efficiency and increasing pollution. With an advanced, medium-sized stove, it is possible to achieve a reliable overnight burn while maintaining flaming combustion and having enough charcoal in the morning to rekindle a new load.

Other Useful Tips

Arranging the firewood: Small pieces of firewood arranged loosely in a crisscross pattern burn quickly because the heat and combustion air can reach all the pieces at once. Larger pieces placed compactly burn more slowly because there are fewer spaces where the air can penetrate the load. Avoid adding just one or two pieces of wood to a fire at a time. Three or more pieces are needed to form a sheltered pocket of glowing coals that reflect heat toward each other and sustain the fire.

Fire in cycles: Don’t expect perfectly steady heat output from a wood fire. Wood fires burn best in cycles. A cycle starts with the ignition of a new load of wood and ends when that load has been reduced to a coal bed. Each cycle should provide between three and eight hours of heating, depending on how much wood was used and how much heat is needed. Plan the firing cycles around your household routine. If someone is home to tend the fire, use a short firing cycle. If you must be away from the house during the day, use the extended firing cycle discussed below.

The flash fire: A flash fire is a small amount of wood burned quickly. Use it in spring and fall when you just want to take the chill off the house. The flash fire technique eliminates the smoldering fires that are common in spring and fall. To build a flash fire, rake the charcoal towards the air inlets and place several small pieces on and behind it. The pieces should be stacked loosely in a crisscross arrangement. Open the air inlet to produce a hot, bright fire. The air supply can be reduced slightly as the fire progresses, but never enough to extinguish the flames. When only charcoal remains, the air supply can be reduced further to prevent cooling the coal bed.

The extended fire: To build a longer-lasting fire, rake the coals towards the air inlets and use larger pieces of wood placed compactly in the firebox. Placing the pieces close together prevents the heat and flame from penetrating the load and saves the buried pieces for later in the burn cycle. Open the air inlets fully for between five to 20 minutes depending on load size and fuel moisture content. When the firebox is full of flames and the outer pieces have a layer of charcoal, reduce the air control in stages to the desired level. The charcoal layer insulates the rest of the wood and slows down the release of combustible gases. This allows you to turn down the air control and still maintain a clean burning fire. Use the extended fire technique to achieve an overnight burn or a fire to last the day while you are at work. The wood should be actively flaming even after the air setting is reduced for the extended burn.

Removing ashes: When you follow the suggestions for raking of the coal bed before loading, you will find that ashes build up at the front of the firebox. These ashes can be removed easily before coal bed raking in preparation for loading. Most modern wood-burning appliances work best when a small amount of ash is removed often rather than letting it build up for several days. Many advanced stoves have ash pans below the firebox and various ways to let the ashes fall into them. Some have grates, and others have removable plugs. When these ash removal systems are closed and the stove is operating, no air should leak past the grate or plug. If the coal bed glows at this location, the ash door gasket may need replacing or the ash pan plug may not be fitting properly. These problems should be corrected so the stove can operate as intended.

If your stove does not have an ash pan or if you find the one you have awkward to use, ask your wood stove retailer for an ash handler that you can scoop ashes into through the loading door. Ashes almost always contain live embers that can stay hot for days and which release carbon monoxide gas. Ashes should be placed in their own metal container with a lid that is stored outside on a surface like concrete. Never store ashes indoors, in a nonmetallic container or on a wooden surface like a deck.

Using a thermometer: A stack thermometer was an important indicator of the condition of fires before glass doors with air wash were perfected. With no view of the fire, the thermometer was an important way to know that the wood had ignited properly and wasn’t smoldering. If your appliance doesn’t offer a view of the fire, a thermometer might be helpful to you. Every wood-heating system behaves differently, and thermometers vary, so it is not possible to give exact temperature guidelines. With some experience you will be able to tell when you have set the air control too low for a long burn and when it is a good time to reload. Install the stack thermometer in the flue pipe about 450 mm (18 in.) above the stove. Some appliance manufacturers recommend the use of a stove-top thermometer rather than a stack thermometer. The function is the same, but the temperature range will be different. Almost all advanced technology stoves have glass doors with air wash, so you can check the condition of the fire easily without a stack thermometer. If the fire is burning properly the glass might get hazy after a few days of use, but it should not develop large brown stains quickly. If the glass does develop stains quickly, you may be turning the air control down too much, or your wood may be too wet. Brown stains on the glass usually indicates that the fire is burning dirty, which also means creosote deposits can be building up in the chimney. Hearth retailers sell a special cleaner for this ceramic glass.

When you burn wood properly, this is what you should see:

When wood burns it should be flaming until only charcoal remains. If there are no flames, something is wrong.

If there are firebricks in the firebox, they should be tan in color, never black.

Steel or cast-iron parts in the firebox should be light to dark brown, never black and shiny.

With seasoned wood, correct air settings, and proper loading arrangement, you should expect almost instant ignition of a new load of wood.

If the appliance has a glass door with air wash, it should be clear.

If the appliance has a glass door without air wash, it will be hazy, but should never be black.

The exhaust coming from the top of the chimney should be clear or white. A plume of blue or grey smoke indicates smoldering, poor combustion, air pollution and probably low system operating temperatures.